Choose wisely, as Barack Obama did in 2008, and you get a safe, mildly re-assuring running mate like Joe Biden.

Though long-winded and prone to the knee-slapping gaffe, Biden did what he was supposed to do — ease anxiety about a Democratic candidate lacking experience both in Washington and on the foreign policy front.

Choose poorly, as John McCain did, and you get a vice-presidential train wreck.

If Americans have learned anything about Mitt Romney through the 2012 presidential cycle, it is that he is no John McCain.

There will be no Sarah Palin-like “game change” moment leading to the Republican national convention in August — it’s just not in Romney’s DNA to gamble away his long-sought shot at the White House.

But that’s not to say there won’t be intrigue as he winnows his list of potential running mates.

Last week, Florida Sen. Marco Rubio set tongues wagging when he made a campaign appearance with Romney in Pennsylvania — an event deemed to be his unofficial tryout for the second slot on the GOP ticket.

But other names abound, from the highly unlikely, such as Rick Santorum, to the mostly unknown, such as Ohio Sen. Rob Portman.

Here’s what we do know about Romney. To win in November, he needs to succeed in specific geographic areas — swing states from Florida to Ohio to New Hampshire — and desperately needs to improve his standing among female and Latino voters. Moreover, he could stand help shoring up support on the Republican right.

Here’s a subjective look at the folks who fill one or more of those needs:

Marco Rubio: If Vegas bookies were laying odds on the most likely Romney veep, this 40-year-old Cuban-American senator from the Sunshine State would be the 3-2 choice..
Rubio is the rising star of the Republican party, elected in Florida as a favourite of small-government Tea Party activists. His fans believe putting Rubio on the ticket delivers Florida and its 29 electoral votes to Romney. He may also help Romney close the yawning gap between him and Obama among Latinos. Rubio recently offered up an immigration-reform plan that would create a path to legal status for children of undocumented immigrants. Downside: At 40, he’s still wet behind the ears. And a new biography claims his Cuban-born grandfather once faced a deportation order.

Rob Portman: He’s been dubbed the “Vice-President Vanilla” by Politico, and the nickname fits. Portman, 56, is a super-low-profile, quietly effective senator who served as director of the Office of Management and Budget under president George W. Bush. He’d be on the ticket because of his potential to tilt Ohio — the state that decided the 2004 election — toward Romney. Portman’s budget experience might also bolster Romney’s argument that he’s best suited to handle the nation’s finances. Downside: The George W. Bush economic record is a heavy burden.

Gov. Nikki Haley, Sen. Kelly Ayotte and Gov. Susana Martinez: Romney is facing a 29-point deficit to Obama among women, and needs a lifeline. Haley was a GOP rock star when she became South Carolina’s first female, first Indian-American governor in 2011. Conservatives love her for battling the U.S. Labour Department when it tried to deny South Carolina a Boeing manufacturing plan because of her state’s right-to-work status..
Ayotte, 43, a rookie senator from New Hampshire, is mother of two who might put Romney over the top in the Granite State..
Martinez, governor of New Mexico, could appeal to both female and Hispanic independents. Downside: South Carolina is going to vote Republican anyway, so Haley offers little strategic benefit. Ayotte and Martinez, 52, have no national profile.

Paul Ryan: Perhaps the riskiest choice in the bunch. The Wisconsin congressman, 42, is chairman of the House budget committee. Conservatives love for him for daring to talk about cutting food-stamp programs and privatizing Medicare for retirees, but he is the White House crosshairs for the same reasons. Even though Wisconsin may be in play in 2012, Ryan could be too polarizing for moderate voters there, and elsewhere.

Chris Christie: The 49-year-old New Jersey governor certainly has the “wow” factor. But he would risk upstaging Romney at every turn, and New Jersey isn’t voting GOP in 2012. Downside: Strategically, Christie does next-to-nothing for Romney.

Condoleezza Rice: The former secretary of state topped a recent CNN/Opinion Research vice-presidential poll among Republicans. But name recognition wears thin fast on the campaign trail. Downside: Again, George W. Bush.